1. Field of the Invention
This invention pertains generally to radiation shields and more particularly to a flexible radiation shield that can fill an opening in a thermal expansion gap and accommodate the varying clearances within the gap with temperature without a loss of shielding integrity.
2. Description of the Related Art
In a typical nuclear power plant, most notably those that employ pressurized water reactors, the reactor pressure vessel utilized to generate heat, as well as other components such as steam generators, pumps, pressurizers, and associated piping, are housed in a containment building. Typically, a containment building may be made of concrete, stainless steel, a combination of the two, or other appropriate material. The containment building defines a refueling cavity and completely encloses the entire reactor and reactor coolant system and ensures that an acceptable limit for release of radioactive materials to the surrounding/local environment would not be exceeded, even in the unlikely occurrence of a gross failure of the reactor coolant system. All operations and procedures associated with the functioning of the reactor vessel and the reactor coolant system are performed within the containment building.
Typically, a refueling cavity is provided for in the containment. The refueling cavity is generally a split level area, wherein the upper level contains a reactor cavity and the lower level consists of a fuel transfer canal. The reactor vessel is housed within the reactor cavity which is also a reinforced concrete structure. When filled with water for refueling, it forms a pool above the reactor within the refueling cavity. The refueling cavity is filled to a depth that limits radiation at the surface of the water, usually up to an operating deck from which maintenance procedures are conducted, to acceptable levels. Typically, the water is in the form of borated water, which helps to minimize radiation exposure levels. The water provides an effective and transparent radiation shield for personnel on the operating deck, as well as a reliable medium for the removal of decay heat from the reactor vessel. During refueling, spent or depleted fuel is removed from the reactor core, transferred under water, and placed in a refueling transfer system by the plant's refueling machine; and new or fresh fuel is similarly transferred from a fuel transfer building outside of the containment through a fuel transfer tube within the fuel transfer canal, for loading in the reactor. After the refueling operation is complete the water is drained from the refueling cavity and the refueling canal and the fuel transfer tube is sealed within the containment before the reactor is started up.
During refueling and maintenance operations every effort is made to shield personnel from radiation exposure. During shield design activities for a new generation of nuclear plants, a location was identified that was difficult to shield by conventional methods. This area was a two-inch wide expansion gap between the reactor containment and the concrete shield around the fuel transfer tube where it extends from the fuel handling building through the containment. This gap can vary depending on temperature conditions inside and outside the containment; requiring a shield that can accommodate this variability. This is an important issue, since overexposure of plant workers have occurred in the past due to such gaps in shielding.
Gaps in or between radiation shields can result in highly localized radiation fields outside the gap that may not be readily detected by radiation protection personnel. This problem is exacerbated by situations in which the radiation source that is being shielded is not fixed and is intermittently introduced behind the shield wall. An example of this is the expansion gap around the fuel transfer tube in a nuclear power plant. Suitable space must be maintained between the outside steel containment shell and concrete shield material that is placed around the fuel transfer tube. A gap that is typically two inches wide is required to accommodate thermal expansion of the containment vessel. Also, for a concrete containment with a steel liner, seismic gaps that are typically two inches in width are provided between the containment wall and/or steel liner and the transfer tube shielding. When a spent fuel assembly is transferred from the containment to the spent fuel pool within the fuel handling building, through the transfer tube; the dose rates outside the gap can result in potentially lethal doses of radiation to personnel. Several instances of worker overexposure have occurred in the past resulting in nuclear regulatory commission instructions to licensees that requires strict control of access and posting (for example, see Nuclear Regulatory Commission Office of Inspection and Enforcement Bulletin No. 78-08, “Radiation Levels From Fuel Element Transfer Tubes,” Jun. 12, 1978).
Various “shadow shield” type shielding arrangements using lead or steel have been considered, but have been rejected due to the cost and difficulties in their design and installation.
Accordingly, a new means of shielding is desired that can affectively protect personnel while accommodating the variations in expansion gaps.
Furthermore, such a method of shielding is desired that will last over extended outages and preferably for the life of the plant.